Zircon ore contains zirconium, as its major component, and hafnium (generally from 1 to 3% by weight). For its use in nuclear power, after carbochlorination of the ore, zirconium has to be purged as far as possible of hafnium, which is therefore found in the residual fractions of the processes for the purification of zirconium. The hafnium may, in turn, be used, and this necessitates the individual purification thereof.
For example, one of the most effective processes currently used for the purification of zirconium is that known as the process for separation in molten salts or extractive distillation in molten salts (FR-A-2 250 707 and FR-A-2 629 360). This process uses a distillation column having a plurality of plates, each supporting a layer of molten salts. A mixture of ZrCl4 and HfCl4, which is derived from the carbochlorination of the zircon ore, is introduced into the column in a gaseous state. A ZrCl4 fraction is recovered in the solvent phase at the base of the column, whereas a residual fraction enriched with HfCl4 is produced in the vapour phase at the top of the column. This residual fraction may thus, for example, comprise approximately 70% ZrCl4 and 30% HfCl4 (% by mass). An industrial plant operating in accordance with the principle may be reconditioned in order to treat this residual fraction and recover the hafnium, resulting in discontinuous operation of the plant.
Moreover, the demand of specific industries for ever-more-pure hafnium has also created a need for a process allowing large amounts of very pure hafnium to be purified.